C4 Corvette Tuning - Late Flash For L98s

Weswood Performance's Digispark System Brings The Ease Of Flash Programming To L98 Engines

When electronic engine controls first appeared, they seemed to be sounding a death knell for the kinds of power-enhancing modifications Corvette owners had come to know and love. But by the late 1980s, pre-programmed performance PROMs (Programmable Read Only Memory) and do-it-yourself software became readily available and it was once again "game on" for high performance engine modifications and tuning.

The PROMs (actually EPROMs, the "E" means erasable) used in the ECMs (Engine Control Module) that control L98 and 1992-93 LT1 engines are removable and reprogramming them requires that they first be erased using ultra-violet light. Once a PROM's data has been erased, it can be programmed using a "burner", but the process doesn't always move to completion as planned. Sometimes existing data can't be completely erased, and other times what was a perfectly good PROM, won't allow new data to be written.

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Although the '91 test car has stock exhaust manifolds, it's been equipped with a Random Technology SuperStainless catalytic converter and custom Y-pipe. These parts were installed before Digispark testing began, to assure that exhaust restrictions wouldn't put a crimp in the horsepower curve.

By comparison EEPROMS (electronically erasable programmable read only memory), also called "flash" memory require neither ultra-violet light for erasing existing data nor a special PROM burner to write new data. Erasing and writing are both done electronically, which is one of the reasons EEPROMS can be repetitively re-flashed without problems.

Until recently, an uncooperative PROM was simply a minor inconvenience that could be eliminated by purchasing a new one. A few years ago, what was a minor inconvenience turned into a major pain in tender parts of the anatomy. First GM switched to non-erasable PROMs, and then they eliminated them completely from the replacement parts inventory.

Difficulty in obtaining replacement PROMS combined with limitations in the original control systems prompted Weswood Performance to develop an entirely different solution to custom programming L98 engines. Weswood's Digispark system uses a 1994-95 LT1 PCM (Powertrain Control Module) in place of the original ECMs used in 1985-1993 Corvettes. As opposed to a complete control system replacement, the Digispark system uses the LT1 PCM to control engine (and transmission, if desired) functions, but retains the original ECM to control all emissions equipment and handle communications with the body computer, which in turn operates the digital portions of a C4 dashboard.

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The key to interfacing an LT1 PCM to an L98 engine is the patent-pending Digispark distributor. It features an OptiSpark shutter wheels and sensor mounted in a conventional HEI housing.

Obviously, you don't fall off a turnip truck one day and design a system like this the next. On the other hand, if, like Wes Burch of Weswood, you're a retired electronic engineer with 30-plus years of experience designing sophisticated electronic systems for the US Navy, building a custom engine control system is a relatively straightforward task--but definitely not an easy one. General Motors doesn't disclose many of the deep, dark details of engine control system functions, so Burch had to do a fair amount of research to determine the precise electronic details needed to assure proper operation and compatibility.

Burch installed the prototype Digispark system in January, 2008 on his '72 Corvette, in which he had previously installed an L98 engine using a '90 ECM and "mail order" chip. After he had the Digispark system operating satisfactorily, he built another system and installed it on his '91. He's been using both systems as long term validation tests to verify that no durability problems exist, and to explore the capabilities of off-the-shelf tuning software. To date the two cars have accumulated over 10,000 trouble free miles including several heat-soaked 400-plus mile trips and a few drag strip passes.

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C4 Corvette Tuning - Late Flash For L98s

Although the '91 test car has stock exhaust manifolds, it's been equipped with a Random Technology SuperStainless catalytic converter and custom Y-pipe. These parts were installed before Digispark testing began, to assure that exhaust restrictions wouldn't put a crimp in the horsepower curve.

The key to interfacing an LT1 PCM to an L98 engine is the patent-pending Digispark distributor. It features an OptiSpark shutter wheels and sensor mounted in a conventional HEI housing.

The Digispark/LT1 PCM mounts in the space formerly occupied by the original ECM...

...which moves to the front left corner of the engine compartment. The black box just above the ECM is the Digispark interface module.

Whether an EFI system is of the batch fire or sequential persuasion, the plug wires have to be connected in the proper sequence. But with sequential fuel injection, the injectors must also be connected in the correct sequence, hence the numbers on the lead wires.

The guts of the "little black box" exposed. Like a marriage counselor, the Digispark interface module keeps the original ECM and the LT1 PCM on speaking terms.

The Digispark Test Control Center--also known as the interior of Wes Burch's '91 convertible. In addition to the existing instruments, Burch installed an Innovate Wide Band system and a few other gauges. Fuzzy dice serve as an "analog" accelerometer.

Aside from the LT1 PCM, and the relocated original ECM, there isn't much evidence that engine control is being handled by a Digispark system.